Information Hidden in Light Could Aid Astronomers

October 4, 1994

By exploiting a feature of light they discovered only a few
years ago, University of Rochester physicists believe they've
found a way to reduce the size and cost of many astronomical
experiments. Scientists in India recently reported in the journal
Optical Engineering that they have verified the theory using
small light sources in the laboratory.

Emil Wolf, Wilson Professor of Optical Physics at the
University of Rochester, and former student Daniel James, now at
Los Alamos National Laboratory, will discuss their most recent
work in this general area at the annual meeting of the Optical
Society of America Friday, Oct. 7 in Dallas.

The new technique, which Wolf and James call space-coherence
spectroscopy, would replace with just two quick measurements a
tedious set of measurements astronomers now must obtain to learn
about some celestial objects.

"The technology would give comparable results to what
astronomers are doing now, but for a much lower cost," says
James. Scientists would need less time and far fewer telescopes
to do the same experiments, freeing up expensive equipment for
other tasks, he says.

Wolf and James believe the technique could be used to
observe thermal sources, such as stars and galaxies, but perhaps
not non-thermal sources, such as quasars and pulsars. They say
the technique would work for sources with broad-band radiation,
which most astronomical sources have. Although the radiation
reaching the earth is usually not broad enough because the
atmosphere blocks out many colors of light, orbiting telescopes
routinely detect broad-band radiation and could be equipped to
take advantage of the finding, says Wolf.

While the correctness of the theory has been demonstrated in
the laboratory, James says, "it's a big leap from the laboratory
to astronomy. Even though our predictions have been verified in a
nice clean laboratory, the real world is a different matter.
There can always be experimental difficulties that you can't
foresee."

The work is a direct result of a property of light
discovered by Wolf in 1986 and now widely known as the Wolf
Effect. Then Wolf found that the way in which atoms in a source
are ordered affects the way they emit light and the way the light
travels through space. For example, in a laser the photons are in
step with one another and are said to be "coherent." In contrast,
the photons from a light bulb or candle flame are emitted
randomly and are incoherent. Light in between -- where some of
the photons are in step and others aren't -- is partially
coherent light, a subject about which Wolf is widely known in the
optics community as the world's expert. It is this type of light
that can cause spectral shifts.

Wolf and James say that two measurements of the spectrum of
a source that emits partially coherent light tells how the
photons emitted by that source are in step, giving a measurement
known as the light's "correlation," from which astronomers can
calculate the size and other features of the source. Detecting
correlation of sources currently requires a long set of
experiments.

The technique would be used in stellar interferometry, which
scientists use to detect very small or faint stellar objects.
Stellar interferometers typically consist of anywhere from two to
a few dozen telescopes or mirrors scattered miles apart that
effectively form one giant telescope by collecting light from a
wide area and combining the signals to piece together the stellar
source. Such facilities include the National Radio Astronomy
Observatory's VLA, a group of 27 telescopes scattered across the
Plains of St. Augustine in New Mexico, and the Australia
Telescope, an array of 12 telescopes concentrated in New South
Wales.

"We obtain information from all the frequencies at once,"
Wolf explains. "With current stellar interferometry astronomers
must cut out most of the radiation because they work with a very
narrow band of it. We look at all the radiation."

Wolf and James believe that by using the new information,
the number of telescopes astronomers need to make certain
measurements falls dramatically.

Some astronomers, however, are skeptical about some aspects
of the theory because one feature of the proposed technique was
discovered 20 years ago and is embodied in a formula known as the
space-frequency equivalence theorem.

But James and Wolf say the objections are based on an
incomplete understanding of their work. The Rochester team says
that the idea of measuring spectra to determine the correlation
between multiple broad-band signals of an interferometer is
completely new. Using the old formulation, astronomers took
measurements at hundreds of frequencies, one at a time; with the
new method, just two readings of the spectrum yields the same
information.

"It's much more rapid and efficient," says James.

Since Wolf first announced the result known as the Wolf
Effect, other scientists have raised objections which he has
shown to be irrelevant. Wolf and James have demonstrated in
several papers (most recently in a May issue of Physics Letters
A) that conditions commonly found in atmospheres can shift light
spectra, and he believes the theory may explain some inconsistent
quasar data. But Wolf remains skeptical of scientists who have
adopted his work as evidence that the universe is not expanding
(expansion is generally accepted among physicists).

The work by Wolf and James is finding other uses. Japanese
scientists are using it to explore basic properties of light, and
some astronomers are using it to study gravitational lensing. And
Wolf believes the ability to modulate coherence properties of a
beam may make it possible to develop a new technique for signal
coding.

"Instead of FM (frequency modulation), AM (amplitude
modulation), or PM (phase modulation), why not CM (coherence
modulation)?" Wolf asks. "People are used to thinking of the two
extremes, either completely incoherent light such as generated by
a light bulb, or coherent light such as produced by a laser. But
there is a whole world in between," says Wolf, whose work is
funded by the Department of Energy, the U.S. Army, the National
Science Foundation, and the New York State Science and Technology
Foundation.
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